Cancer 1 Unregulated cell growth Family of >100 related diseases Cells multiplying out of control Tumor: mass of cells from repeated cell division A tumor can be benign (not called cancer): Grows slowly Tissue remains differentiated Enclosed in a capsule A benign tumor can still be dangerous E.g. a brain tumor, slowly crushes brain
Cells from tumor spread elsewhere: metastasis Tumor that metastasizes = malignant Cancer cells are different In culture, are rounded and float rather than stick to surfaces. Become de-differentiated Immortal Do not respond to cell signals Do not stop growing when crowded or touching No contact inhibition No dependence on normal growth signals Loss of normal number and structure of
chromosomes 2 Types of cancer Cancer is named from the type of tissue in which it arises. There are various historical classifications. Carcinomas: from epithelial tissue; 90% of all cancers. Leukemias, lymphomas, and myelomas: from bone marrow and blood cells; 8% of all cancers. Sarcomas: solid connective tissue such as muscle and bone; 2% of all cancers. Cells that multiply often are more likely to form cancers. http://www.willamette.edu/~stas/physiology/labs/lab1/epithelial2a.jpg
3 Cancer and aging As people age, more mutations accumulate, increasing the risk of cancer. Also, as cells age, they too can become cancerous. http://ovid.iss.it/html/lecture/pc0161/img003.GIF
4 Cancer is a genetic disease 5 Predisposition to cancer can be inherited Breast cancer, retinoblastoma, Li-Fraumeni Clonal expansion: all cells in a tumor descend from the abnormal cell, all inherit bad genes. Agents that damage, mutate DNA cause cancer Chemicals, UV and ionizing radiation
Certain genes brought into DNA by viruses cause cancer Cancer cells show chromosomal loss, damage Review: The Cell Cycle G1: a time of cell growth and general functioning. S: all the DNA in the cell is doubled to prepare for division. G2: cell prepares for division. M: mitosis, actual dividing up of the copied chromosomes and distribution to daughter cells.
http://www.med.unibs.it/~marchesi/cellcycle.gif 6 Review: Cell cycle, continued G0: cells not dividing; may never divide again, or may re-enter cycle when needed. Some cells divide nearly continuously; others enter G0 once mature. 7
Cell cycle (continued) From cancer research, we have learned that the cell cycle is tightly regulated! Checkpoints exist: at G1/S, at G2/M, and late in Mitosis (the M checkpoint) At each checkpoint, progress evaluated. G1/S checkpoint: is DNA in good condition? Mutations that allow progression past this checkpoint with damaged DNA lead to downward spiral of inherited mutations. 8 Molecular regulation
Two kinds of proteins work together: cdc kinases and cyclins Kinases: proteins that phosphorylate other proteins Adding a PO4 turns molecules on or off Kinases always present in the cell Cyclins are proteins that change in amounts during the cell cycle Specific cyclins accumulate at different times. Kinase combines with cyclin Kinase is activated, given directions Kinase phosphorylates proteins controlling cell division 9
Kinase/cyclin combination acts during checkpoint G1/S checkpoint important in cell cycle regulation. Important tumor suppressor genes involved in regulation at this step. Picture based on Hartl & Jones, 5th edition. 10 The multi-hit hypothesis
11 Cancer results from at least 2 hits, 2 instances of damage to the DNA. Some cancers involve several. The gas pedal/ brake analogy For the car to move, gas pedal doesnt do much if brake is on; taking brake off doesnt get car to move if gas pedal isnt pressed. Cell must get ON signals AND ignore OFF signals On: growth factor stimulation Off: tumor suppressors
New insights into off/on regulation 12 Recent research: effect of estrogen on entire genome of ovarian tissue Estrogen is a growth factor, promotes cell division Genes for promoting cell division AND stopping cell division both turned on: a balance Like riding the brake to keep car under control Genetics of on and off 13
On signal mutations are dominant for cancer If other mutations present, one mutated copy of the gene is enough for cancer. Off signal mutations are recessive for cancer. But are dominant for a susceptibility to cancer. Both copies of tumor suppressor genes must be bad for cancer to occur. Familial predispositions: usually one copy is already mutated, much more likely to get a mutation in one copy of the gene than both. Genetics visualized
14 What are the on and off signals? 15 Off signals: tumor suppressor genes Genes (and their proteins) which act to prevent the cell from proceeding through the cell cycle. pRB, p53, BRCA1, BRCA2 On signals: proto-oncogenes Genes that stimulate cell division. Mutated forms called oncogenes Through various malfunctions, stimulate cell
division when inappropriate. Tumor suppressor genes Retinoblastoma Cancer of the retina of the eye Occurs in about 1 in 15,000, usually ages 1 3. 40% inherited susceptibility; 60% sporadic RB1 gene codes for pRB, a tumor suppressor Functions at the G1/S checkpoint Protein is active during G0 and G1 Binds to E2F, a transcription factor Prevents E2F from working, prevents transcription of genes that aid cell division.
16 17 RB continued At G1/S, cyclin complex phosphorylates pRB, inactivating it and releasing E2F. E2F is a transcription factor controlling 30 genes needed for progression through cell cycle. Mutation in both copies of pRB allows unregulated passage through G1/S checkpoint.
p53: the Guardian of the Genome 18 An important tumor suppressor; various signals including DNA damage activate p53 which then Activates DNA repair Blocks cell division Prevents replication of damaged DNA Triggers apoptosis in damaged cells. Cells beyond repair are killed to avoid mutations Mutations in p53 account for >50% of cancers. Single mutation is dominant negative because of
tetrameric structure. Action of p53 Continually made but continually degraded Also Inactive because NOT phosphorylated DNA damage activates protein kinases Results in phosphorylation (thus activation) Degradation of p53 stops. Active p53 accumulates in cell, carries out its duties http://www.accelrys.com/webzine/01/q1/appnotes/geneatlas_japan/p53.jpg 19
BRCA genes 20 10% of breast cancers involve a familial susceptibility; mutations in BRCA1 or BRCA2. Woman with a faulty BRCA1 allele has an 85% chance of developing mutation in 2nd allele, thus getting breast cancer (with risk of ovarian cancer) Proteins coded for by BRCA1 and 2 in nucleus Abundant during S phase Work with other proteins in DNA repair, etc. Failure of these proteins to repair may result in the
mutations that lead to cancer. Cancer continued 21 Proto-oncogenes to oncogenes Proto-oncogenes are normal cell genes that turn on cell division. A mutated or over-expressed proto-oncogene becomes an oncogene: gene leading to cancer Given other appropriate mutations: These are dominant mutations for a cancer phenotype; that is, only one bad copy is enough.
Two gas pedals: one turned on makes it go. Control of cell division Proto-oncogene proteins found in membrane, cytoplasm, and nucleus. 22 Cancer results from too much ON Effect of mutations: Cancer cells produce their own growth factors. Increase in growth factor receptors.
ON signal sent to nucleus no matter what. Example: ras, described in text. Mutation in transcriptional activator; wrong genes turned on. 23 Oncogenes: the rude surprise 24 Peyton Rous, 1966 Nobel Laureate Discovered that viruses could cause cancer in chickens (e.g. Rous sarcoma virus)
Molecular biologists identified genes in the viruses that were responsible: oncogenes. DNA hybridization studies revealed: we all have comparable genes: proto-oncogenes. In human cancers, viruses can bring oncogenes into our DNA, or changes can occur in our DNA without virus involvement. How oncogenes are made 25 Point mutations Ras is a intracellular signal peptide Mutation makes it always stimulate cell division
Translocations Burkitts lymphoma: t(8;14) moves myc gene to a more active promoter, drives proliferation. myc is a gene for a transcription factor. Philadelphia chromosome: t(9;22) creates a fusion protein (protein kinase) that activates cell division. How oncogenes are made-2 Over-expression & Gene amplification: too many copies of receptors, internal signalling molecules, transcription factors. Example: myc. Transcription factor that causes transcription of genes involved in cell division. Virus insertion may put active promoter next to gene.
Virus may bring in extra copies of gene. Cmyc-FISH www.pathology.unibe.ch/.../ speztech/spez_fish.htm 26 Another overview 27 Historically, several models of cancer have been developed since War on cancer declared by Nixon, 1970. ON/Off switch idea Multi-hit hypothesis, textbook: colon cancer
Gatekeeper and Caretaker genes Gatekeepers: tumor suppressor genes. Prevent cells from dividing when theyre not supposed to. Caretakers: protect the DNA, repair damage. Review of Telomerase Because of discontinuous synthesis of linear DNA, each round of DNA would shorten the chromosome, eventually causing chromosome instability and cell death. Telomerase adds
protective DNA to end, making cell immortal. 28 Cancer and Telomerase Stem cells, frequently, rapidly dividing cells, maintain telomerase. Descendents have a life span. Telomerase gene is shut off, enzyme not made. Lack of telomerase is a cell clock. Chromosome becomes unstable, cell undergoes apoptosis and dies; replaced by new cells. Some cells cheat death.
Cancer cells turn telomerase back on, become immortal. Area of active study. 29 Cancer and the Environment 30 Familial predispositions exist, but sporadic cancers are more common. Environmental factors combine with genetic background to produce cancer. Microbes, chemicals, radiation all implicated.
Direct experimentation not possible w/ humans. Evidence is correlation, not causation. Statistical analysis of epidemiological data Not proved that smoking causes cancer Need statistical evidence plus plausible mechanism Cancer and microbes 31 Retroviruses and other inserting viruses RNA to DNA, inserts into chromosome, directs synthesis of new RNA. HIV, Rous sarcoma virus. DNA viruses that insert: Hepatitis B Hepatitis B infected: 100x more likely to get liver
cancer Epstein Barr Virus, Kaposis sarcoma virus EBV associated with Burkitt Lymphoma (and mono) Herpes virus KSV causes sarcoma in HIV infection Cancer and microbes-2 32 Sex causes cancer: Papilloma virus and cancer of the cervix. Papilloma virus causes warts, including genital warts; sexually transmitted disease. Subtypes, instead of causing warts, inserts into
DNA, causes increased expression of viral protein E7. See next slide. Helicobacter pylori Bacterium now known to cause ulcers. Highly correlated with stomach cancer; under study. Cancer and microbes-3 pRB is a tumor suppressor protein that prevents the action of transcription factor E2F by binding to it. Viral protein E7
bids tightly to pRB, freeing E2F to transcribe genes promoting cell division. 33 http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/E7_Rb.gif Cancer and the environment 34 Relatively few cancers are inherited, most
result from interactions with the environment or lifestyles Tobacco use Benzo-a-pyrene, tar, radioisotopes, formaldehyde Diet: natural and produced carcinogens Aflatoxins, nitrosoamines Fiber, antioxidant compounds helpful. Radiation: UV , ionizing
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